A conventional ball grid array is a two-dimensional array of solder balls or solder bumps formed on a major surface of a substrate such as a semiconductor integrated circuit or a circuit board. Typically, the balls or bumps are mounted on pads on the surface of the substrate; and are in non-rectifying electrical and mechanical contact with the pads. The term “ball grid array” as used herein also includes the pads. For convenience, we will refer to whatever underlies the pads as a substrate. The substrate includes electrical conductors that may extend across the substrate and/or into the substrate to connect the pads to other pads or other circuitry in the substrate. In some cases such as where the substrate is a circuit board, the conductors may extend through the substrate to a second major surface of the substrate.
Solder balls or solder bumps range widely in size depending on the application and the size of the substrate. They have a pitch as measured between the centers of two adjacent solder balls or solder bumps that may be as tight as about 2× their diameter. By way of example, but not limitation, a typical solder ball diameter is about 500 microns.
The solder balls and solder bumps provide non-rectifying electrical interconnects between the substrate and external circuitry for input/output (I/O), power and ground connections. Typically, the external circuitry is mounted on a second substrate such as a wiring board; and the solder balls or solder bumps are in electrical and mechanical contact with the external circuitry on the second substrate. Despite the relatively large number of solder balls or solder bumps that are present in a 2-D array compared to many other interconnect technologies, there always seems to be a need for more input/output connects
At the same time, the solder balls and solder bumps take up a lot of real estate on the surface of the substrate while performing essentially only one function: electrical connectivity.